Sudden Unexpected Death in EPilepsy, or SUDEP, is a leading cause of death in patients with epilepsy. SUDEP mechanisms are not understood, although there is evidence to implicate apnea, autonomic dysfunction, and cardiac arrhythmias. We will take advantage of recent progress in the understanding of SUDEP risk in the genetic epilepsies to investigate the role of cardiac arrhythmias. SUDEP risk varies in a gene-specific manner. Loss-of- function variants in the voltage-gated sodium channel (VGSC) genes, SCN1A or SCN1B, are identified in patients with Dravet syndrome (DS) and gain-of-function variants in the VGSC SCN8A are found in patients with Early Infantile Epileptic Encephalopathy 13 (EIEE13). DS and EIEE13 patients have the highest SUDEP risk, up to 20%. In contrast, variants in chromodomain helicase DNA binding protein 2 (CHD2) are also associated with early onset EE, but SUDEP has not been reported in this population. SCN1A-, SCN1B-, SCN8A-, and CHD2- linked epilepsies are developmental and epileptic encephalopathies (DEEs), severe childhood epilepsies associated with cognitive and behavioral impairments. The familial focal epilepsies, are attributed to pathogenic variants in DEPDC5, encoding a member of the GATOR complex in the mTOR pathway. SUDEP is reported in 10% of these patients. Because VGSC genes are expressed in both heart and brain, we have proposed that cardiac arrhythmias contribute to the mechanism of SUDEP in channelopathy-linked genetic epilepsies. Our overall goal is to understand the mechanisms of SUDEP in the genetic epilepsies. Our objectives are to use patient-derived or transgenic mouse cardiac myocytes (CMs) to understand how epileptic VGSC gene mutations alter CM function and arrhythmogenic potential, and to determine whether similar changes are found in non-ion channel epilepsy genes that are expressed in the heart. Our central hypothesis is that both ion channel and non- ion channel genetic epilepsies with high, but not low, SUDEP risk exhibit pro-arrhythmogenic changes in patient- derived CMs and mouse models. To ask whether abnormal CM excitability also occurs in a non-ion channel genetic epilepsy with high SUDEP risk, we will investigate DEPDC5 variant iPSC-CMs and Depdc5-/- mice. Finally, we will examine Chd2-/- mice and human iPSC-CMs with variants in CHD2, a non-ion channel gene with a low SUDEP risk, to test whether altered CM excitability is specific to genetic epilepsies with high SUDEP rates. Like the VGSCs, DEPDC5 and CHD2 are expressed in brain and heart. Our Specific Aims are: 1. To determine the effects of SCN1A, SCN1B, and SCN8A epilepsy variants on CM excitability using patient-derived iPSC-CMs. 2. To ascertain whether CMs from DEPDC5 patients or Depdc5+/- mice display abnormal excitability and whether Depdc5+/- mice have arrhythmia. 3. To determine whether CMs from CHD2 patients or Chd2+/- mice display abnormal excitability and whether Chd2+/- mice have arrhythmia. There are no effective therapies for any of the genetic epilepsies and no reliable biomarkers for SUDEP risk. This work may lead to the discovery of diagnostic biomarkers for SUDEP risk in the future.